This invention relates to a jamming cancellation system and more particularly to a multiple source jamming cancellation system.
An important consideration in the design of electronic systems such as radar, particularly those for military applications, is the provision of means for recovering the desired signal from a total signal which include noise jamming signals fo higher intensity than that of the desired signal itself. Most noise jammers consist fo an oscillator which is frequency modulated by a noise waveform at a high rate. The rates are high enough to shock excite the victor radar i.f. amplifier so that its output is indistinguishable from true random noise.
Various techniques to cancel jamming signals and thus recover the desired signal have been proposed. One such technique is described in a patent application for a "Jamming Signal Cancellation System", Ser. No. 509,158, filed Sep. 24, 1974 and assigned to the assignee of the present invention.
In that application improved signal detection is achieved by detecting the AM difference frequency produced by the beating of the desired signal with the jamming signal and applying same to one input of a balanced modulator while simultaneously applying the received signal (containing both the desired signal and the jamming signal) to the other input of the balanced modulator whereby the output from the balanced modulator contains the desired signal but not the jamming signal. Actually, the video signal applied to the balanced modulator causes double-sideband suppressed-carrier modulation of the jamming signal. As a result the balanced modulator output contains two frequencies: one is the desired signal; the other is an image on the other side of the jamming frequency and frequency modulated with twice the deviation of the jammer. The image is outside the passband of the receiver and thus, only the desired signal is processed.
This single stage device only removed the strongest jammer signal. If a weaker jammer signal is also present it is preserved in its original ratio to the desired signal. Therefore, the apparent improvement factor can never exceed the ratio of the tow jammer powers and generally is a few db less.
A solution to this problem of multiple jammers is disclosed in U.S. patent application User. No. 589,490, filed Jun. 16, 1975 for "Multiple Source Jamming Signal Cancellation Systems" and assigned to the assignee of the present application. In that application suppression of jamming from multiple sources is achieved by detecting the AM difference frequency produced by the beating of a desired signal and the smaller of the multiple jamming signals with the stronger jamming signals and applying same to one input of a single-sideband modulator while simultaneously applying the received signal (containing both the desired signal and the jamming signals) to the other input to the single-sideband modulator whereby the larger jamming signal is suppressed. The sum and difference outputs of the single-sideband modulator containing the upper and lower sidebands, respectively, of the desired signal plus the smaller jamming signal are each processed by a jamming cancellation circuit to suppress the smaller jamming signal and their outputs combined to receive the desired signal. While this system performs adequately it does require that the multiple jamming signals be of different power levels and degradation of the system occurs as the relative power levels of the multiple jamming signals approach unity.